/*
The first N rows will populate this array of rows in sorted order. When the N +1 row is fetched, it will be compared to the last row in the array. If it would go into slot N +1 in the array, it gets thrown out. Otherwise, it is added to this array and sorted and one of the existing rows is discarded. Your sort area holds N rows maximum, so instead of sorting one million rows, you sort N rows.
*/

select *
  from
(select *
   from t
  order by unindexed_column)
 where ROWNUM < :N;

/*
My question about this SQL statement are as follows:
1. Does order by executed on the last order(even though it's a subquery's order?)
2. Why this way just sort N rows? how could get the right answer if it does not sort the entire records?
*/